Single-crystal silicon carbide can be manufactured according to the Lely method by means of sublimation without the use of seed crystals. This technique allows silicon carbide to grow from a partially dissociated compound in an atmosphere enriched in carbon and silicon. Using this method, industrial silicon carbide is decomposed and single-crystal silicon carbide is grown at an elevated temperature of about 2500.degree. C.
According to a modified Lely method, as generally disclosed in DE-PS 32 30 727, single crystals of silicon carbide can be grown from the gas phase on seed crystals of silicon carbide at temperatures of 2100.degree. to 2300.degree. C. This technique results in a temperature gradient that is less than 25.degree. C./cm in the reaction vessel in the direction of growth. In addition, the partial pressure of the protective atmosphere is adjusted during crystal growth so that it is at least equal to the sum of the partial pressures of the components.
The industrial silicon carbide powder used in these methods is typically manufactured according to the Acheson method, generally described in Philips Research Reports, Vol. 18, No. 3, June, 1963, pp. 171-174. In this technique, a mixture of silicon dioxide (SiO.sub.2), carbon, and other additives are placed in a crucible and heated in an oven to a temperature of 2700.degree. C. A graphite core arranged in the middle of the crucible is used as a heating element. By adding salt, various impurities are converted into gaseous chlorides, which subsequently escape from the silicon carbide. This process produces relatively pure silicon carbide.
This industrial silicon carbide powder, however, is not pure enough for electronic components since it still contains traces of impurities such as heavy metal silicides or carbides. These impurities are not uniformly dispersed throughout the silicon carbide, but instead are retained as impurity clusters in a second phase. When single crystals are subsequently manufactured, impurities, whose solubility limit has been exceeded in the crystal, can be precipitated. The silicon carbide powder used as a base material for single crystal growth should, therefore, undergo yet another chemical purification step to lower the concentration of impurities.
To chemically purify the industrial silicon carbide powder used as a base material, acid or alkaline solutions can be applied. One such solution comprises a mixture of hydrofluoric acid (HF) and nitric acid (HNO.sub.3). After being purified, the silicon carbide is rinsed in a liquid that is free of acid and alkaline components, preferably water. The base material is then dried, preferably by heating in a high vacuum.